In arc welding, it is necessary to stabilize a welding arc in order to maintain high quality of the welding. For the purpose, a welding wire has to be accurately fed to a welding station so that a length of the welding arc is constant.
In case that arc welding employs aluminum which is too soft to be conveyed by a push feeder, or utilizes a long passage for feeding the wire, a push-pull welding wire feeder having plural wire feeding units is provided along the passage. The wire feeding unit generally includes a pair of rollers which hold the wire between them and are driven by a driving source, an electric motor.
For steadily feeding the wire with the push-pull welding wire feeder, a pulling speed for pulling the wire at a pull-end feeding unit and a pushing speed for pushing the wire at a push-end feeding unit has to be precisely equal to each other.
In practice, there may however be a small difference in the speeds between the wire feeding units. The small difference is accumulated time to time and finally interrupt the welding.
Japanese Patent Publication No. 50-25425 discloses that two wire feeding units are determined as a master unit and a slave unit, respectively. The master unit is driven by a motor, and is electrically controlled to operate at a constant speed for feeding the wire determined according to requirements of the welding. The slave unit is controlled at constant torque operation.
Japanese Patent Laid-open Publication No. 11-156543 discloses that a push-end feeding unit is equipped with a pneumatic motor which itself has constant torque characteristics for performing the constant torque operation easily.
Each of the conventional push-pull welding wire feeders allows the wire to be steadily conveyed with the slave wire feeding unit controlled at a constant torque, while the wire feeding speed maintained in constant, hence providing proper welding quality.
However, when the wire feeding speed in the conventional wire feeders is periodically changed, e.g. is accelerated and decelerated for producing scale-like welding beads or at the start and end of the welding operation, the master wire feeding unit driven may be loaded irregularly due to a delay in an operation of the slave wire feeding unit. This causes the wire feeding to be hardly consistent, thus making the welding quality deteriorate. Specifically, the slave wire feeding unit controlled at a constant torque causes a force for feeding the wire to be smaller or larger than its desired level since an inertia force in its electric or pneumatic motor as the driving source creates an acceleration torque, i.e., a frictional or viscous torque. This causes the wire to be bent between the master and slave wire feeding units, which is significantly disadvantageous.
The disadvantageous bending of the welding wire often occurs when the wire is jammed or fused down at the tip end of the welding torch. The disadvantageous bending of the wire is caused also-at the slave wire feeding unit located at the push-end to be controlled at a constant torque.
This will result from the facts that the wire is weaker in bending strength than in tensile strength and that the slave wire feeding unit controlled at constant torque operation can hardly stop at once due to the inertia at its driving source.